The present invention is directed to a laser transmitter comprising a semiconductor laser and an external resonator wherein the optical external resonator is in the form of an optical directional coupler which is composed of two light waveguides that extend next to one another with such a slight distance therebetween over a defined coupling length or zone that a power cross-over between the waveguides will occur in the coupling length or zone. The two waveguides and the semiconductor laser are coupled together so that the one waveguide has a feedback device allocated to it so that at least one part of the radiant intensity supplied to the feedback device from the coupling length is conducted back into the coupling length and the output power of the transmitter can be taken from the other waveguide of the resonator which will cause a wavelength selectivity for the transmitter.
A transmitter of this type is proposed in the earlier German Patent application No. P 36 00 726.9, whose disclosure was incorporated in U.S. patent application, Ser. No. 906,503, filed Sep. 12, 1986. In this application, a transmitter is disclosed wherein a feedback means, which is partially transmissive, is allocated to one of the optical waveguides. Part of the radiant intensity which is passed through the feedback means is supplied through a loop-shaped optical waveguide to the other optical waveguide at the one side of the coupling path at which the semiconductor laser is arranged. With such a laser transmitter, the extremely narrow banded, single-mode operating condition can be achieved as required for the future, fiber-optical communications systems, particularly having heterodyne and homodyne reception.
Such a laser transmitter exhibits the advantages that the coupling of the resonator and of the system fiber to the semiconductor laser can occur at only one side of the semiconductor laser in contrast to other known embodiments of laser transmitters where the system fibers are connected to one side of the semiconductor laser while the resonator is coupled to the opposite side of the semiconductor laser. As a result of the one-sided coupling, adjustment problems are considerably reduced and the requirement of a high mechanical stability in the coupling of the system fiber and of the external resonator can be better met.